This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-209579, filed Jul. 23, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an image reading apparatus which reads the image of a document on a document table and performs shading correction on the read image based on a reference signal acquired from a white reference plate.
Generally, a digital copying machine or the like comprises a scanner section for reading an image and a printer section for printing the image read by the scanner section.
The scanner section uses a CCD line sensor for reading an image. The level of the voltage that is stored in the CCD line sensor is greatly affected by a variation in exposure lamp, a time-dependent reduction in illuminance and a variation in the pixels of the CCD line sensor. In this respect, the white level and black level are set immediately before reading an image so that the same image can be copied under any condition. This process is called shading correction.
Such a scanner section uses white and black shading correction plates (hereinafter referred to as xe2x80x9cwhite reference platexe2x80x9d and xe2x80x9cblack reference platexe2x80x9d, respectively), reads signals of several lines for each of the white reference plate and the black reference plate, averages the signal levels to provide reference white and black levels, and corrects a variation in a read image based on the white level and black level. To ensure copying of the same image under any condition, the white reference plate is placed at such a position as to be in the vicinity of the document-reading start position and at the same height as the document surface as much as possible.
Note that the black reference plate is not essential and may be eliminated if the reference black level is generated from a signal obtained when the exposure lamp is turned off, such as the time when the image reading apparatus is powered on.
The following problems arise when the white reference plate in the above-described scanner section is placed in the vicinity of the document-reading start position or at the same height as the document surface.
1. Depending on where the white reference plate is placed, the scanner section cannot be provided with an indicator, a part of the functions of a copying machine, which indicates the scanning area of a document.
2. Because of the restriction on where to attach the indicator, it becomes difficult for a user to determine the document scanning area.
3. The problem 2 is likely to result in miscopying or the like to wastefully consume expendable supplies.
A description will now be given of the problems that arise when the indicator is provided in the scanner section at such a position where a user can easily use (see) it.
1. When the white reference plate is placed at the same height as the document surface, it cannot be placed in the vicinity of the document-reading start position, so that the same image cannot be copied under any condition. This may result in wasteful consumption of expendable supplies.
2. When the white reference plate is placed in the vicinity of the document-reading start position, it cannot be placed at the same height as the document surface. This produces a difference between the white level acquired at the time of scanning the white reference plate and the white level acquired at the time of scanning a document, so that a proper image cannot be copied, resulting in wasteful consumption of expendable supplies.
In other words, when the indicator is placed at the position where a user easily see and use it, the difference between the length of the optical path (optical path length) from the exposure lamp at the time of scanning the white reference plate and the length of the optical path from the exposure lamp at the time of scanning a document produces a difference between the white level of the white reference plate scanned by the CCD line sensor and the white level of the document scanned by the CCD line sensor. This results in a difference between in the density of the output image.
3. In the case of a fast copying machine or the like, the white reference plate is positioned near the document-reading start position in order to shorten the cycle time. Therefore, the white reference plate cannot be placed at the same height as the document surface. This produces a difference between the white level acquired at the time of scanning the white reference plate and the white level acquired at the time of scanning a document, so that a proper image cannot be copied, resulting in wasteful consumption of expendable supplies.
4. In the case where the white reference plate is placed at the same height as the document surface, when dust, an impurity or the like sticks on the white reference plate, a finished image may undesirably have a white line, black line, uneven density and so forth, as mentioned in Jpn. Pat. Appln. KOKAI Publication No. 93783/1998.
Accordingly, it is an object of the present invention to provide an image reading apparatus which has a white reference plate in the vicinity of the document-reading start position, but not at the same height as the document surface, and which can always execute accurate shading correction to provide high-quality images.
To achieve the above object, according to one aspect of this invention, there is provided an image reading apparatus for reading an image of a document placed on a document table by scanning the document using light irradiation means, which comprises a white reference plate located outside a document scanning area near the document table and at a position of a second optical path length between the light irradiation means and the white reference plate, which is different from a first optical path length between the light irradiation means and the document table at a time of scanning the document; setting means for setting a ratio based on a difference between the first optical path length between the light irradiation means and the document table and the second optical path length between the light irradiation means and the white reference plate; change means for changing an amount of light at a time of scanning each of the white reference plate and the document table with the light irradiation means, based on the set ratio; and correction means for, with the amount of light changed by the change means, correcting the read image at a time of scanning the document table, based on a reference signal at a time of scanning the white reference plate.
According to another aspect of this invention, there is provided an image reading apparatus for reading an image of a document placed on a document table by scanning the document using light irradiation means, which comprises a white reference plate located outside a document scanning area near the document table and at a position of a second optical path length between the light irradiation means and the white reference plate, which is different from a first optical path length between the light irradiation means and the document table at a time of scanning the document; setting means for setting a ratio based on a difference between the first optical path length between the light irradiation means and the document table and the second optical path length between the light irradiation means and the white reference plate; photoelectric conversion means for converting an amount of reflected light at a time of irradiating the white reference plate with the light irradiation means and an amount of reflected light at a time of irradiating the document table with the light irradiation means into electric signals; amplification means for amplifying that electric signal which is acquired by the photoelectric conversion means and corresponds to the amount of reflected light from the white reference plate and that electric signal which is acquired by the photoelectric conversion means and corresponds to the amount of reflected light from the document table by different amplification factors based on the ratio set by the setting means; and correction means for correcting the electric signal corresponding to the document table amplified by the amplification means, based on the electric signal corresponding to the white reference plate amplified by the amplification means.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.